In Philly, where Citizens Bank Park (Phillies), Lincoln Financial Field (Eagles) and the Wells Fargo Center (Sixers, Flyers, most concerts) are right across the street from each other, a (pro) baseball game and a (pro) football game are not permitted to take place at the same time.
Baseball and Hockey/Basketball/Concerts at the same time are permitted.
Some of this has to do with parking. The entire complex is basically sized to fit fans going to the 69,000 seat Linc. If there's a Phillies game (45k) and Wells Fargo Event (20k) at the same time, there's still enough parking to hold both events.
The closest this became to being an issue was during the 2008 baseball playoffs, when there was an Eagles game at 1, and the Phillies game at 8.
Last month, there was Phillies NLCS game, Temple Football game, and a Smashing Pumpkins concert occurring at the same time in the complex, but so few people attend Temple games, and the Smashing Pumpkins didn't exactly sell out the Center, so those 2 events barely made a dent in available parking.